Recently, in the field of communications, with the increase in data transmission amount, the data rate has been increased in order to transmit a large amount of data in a single signal. High-speed data transmission has an issue in that degradation of data such as intersymbol interference is likely to occur in cables, boards, output devices, and so forth. Thus, taking the degradation of signals into consideration, an emphasis signal is often used in which a portion where intersymbol interference of the signal is likely to occur is enhanced.
A method (FIR method) for generating such an emphasis signal is disclosed. This method includes splitting a signal into multiple signals, generating a delay difference between the split signals, and adding or subtracting one of the split signals having the delay difference to or from the other signal (see, for example, Japanese Laid-open Patent Publication No. 2004-088693). The use of this method is being considered not only in a communication system that uses electrical signals but also in a communication system that uses light. The use of this method is also being considered in order to compensate for insufficient speeds of laser diodes (LDs) and surface emitting lasers (VCSELs), which are photoelectric conversion devices (see, for example, Japanese Laid-open Patent Publication No. 2012-044396).
However, there is a low degree of freedom in the shaping of the waveform of an emphasis signal generated through the FIR method. Thus, as compared to a case where an emphasis signal is not used, an eye opening improves, but frequency dependence of phase characteristics such as group delay increases, and thus jitter may disadvantageously increase. To resolve such an issue, increasing the number of split taps may be considered, as discussed in Japanese Laid-open Patent Publication No. 2012-044396. However, increasing the number of taps leads to other issues such as an increased circuit size and increased power consumption.